Posted on by

BLUEsat’s High-Altitude Balloon is equipped with a remote separation mechanism. Its function is to detach the payload from the balloon when we want, which gives us better control of the balloon. In the case of problems during the launch (e.g. unexpected changes in weather conditions), the ability to force the payload to separate gives us the chance to drop the payload. This article will explain the construction and operation of the separation mechanism.

The following flowchart describes the Separation Mechanism:

System flowchart of the seperation mechanism for the High Altitude Balloon. It begins with recieving a DTMF signal via an SDR Dongle on the Raspebry Pie. This can then trigger the circuit to cut the rope.
System Flowchart

The operation of the separation mechanism can be summarised as:

  1. Send a radio wave signal (433MHz) with DTMF signal
  2. The separation mechanism system receives the radio signal
  3. Nichrome wire heats up and burns rope after receiving signal.

The mechanism is split into two main modules:

  • DTMF Detection (3 blocks on the left)
  • Rope Burning Electronics (5 blocks on the right + Raspberry Pi Zero)

IMPORTANT: Read, understand and follow the laws regarding radio transmission for your country + area before attempting to transmit. There may be regulations regarding transmission output power, interference, frequency use.

DTMF Detection

The DTMF detection subsystem, as the name implies, detects radio waves with a DTMF signal embedded in. It encompasses the following concepts:

  • Reception of radio waves
  • Conversion to computer data
  • Processing of that data

Background Information

DTMF

Dual Tone Multi Frequency Table. The combination of frequencies represents a number of the chart.
DTMF Table

Above is the DTMF table. By playing one of the Upper and Lower frequencies together, you can represent one of the following numbers/symbols. For example: to indicate 1, play a 697Hz and 1209Hz sine wave at the same time.

We send a DTMF sequence of numbers via radio waves (frequency modulation of 433MHz carrier wave with DTMF tones). If the DTMF sequence is correct, the output GPIO pin (Input/Output Pins) on the Raspberry Pi will change.

Equipment on Balloon:

  • Raspberry Pi w/ Portable Power supply
  • Software Defined Radio (SDR) USB
    • Such as the: NooElec NESDR Mini SDR & DVB-T USB Stick
  • Antenna (ideally receives 400MHz)

Software on the Raspberry Pi:

Equipment on Ground:

  • Radio Transceiver
  • Antenna for transmission
  • Lead Acid Battery
DTMF detection flowchart
DTMF detection flowchart

How it works:

  1. The SDR USB outputs 8-bit I/Q samples, which represent sine and cosine waves. librtlsdr allows for the SDR USB to send these I/Q samples to the host computer (the Raspberry Pi).
  2. These I/Q samples are then processed by rtl_fm (which is part of the librtlsdr package) which demodulates the signal and outputs sound data.
  3. The sound data is then processed by multimon-ng which decodes the DTMF signal (if present). If there is a DTMF signal it will output a character corresponding to the number.
  4. The character output is passed into the dtmf_separation code, which alters the output GPIO pin voltage accordingly.

Rope Burning Electronics

The Rope Burning Electronics takes in the GPIO pin output from the DTMF Detection subsystem and outputs a voltage to the Solid State Relay terminals. In the current configuration the Rope Burning Electronics can be summarised as a switch where:

  • When the GPIO pin is on (3.3V), the Solid State Relay is closed (electrical connection exists)
  • When the GPIO pin is off (0V), the Solid State Relay is open (no electrical connection)

Equipment on Balloon

  • DTMF Detection System (from above)
  • Separation Mechanism PCB (pictured below)
  • Lithium Ion Battery
  • Solid State Relay
  • Nichrome Wire
CAD Render of the custom PCB used by the seperation mechanism.
Separation Mechanism PCB

Background Information

N-Channel MOSFET

The MOSFET circuit diagram symbol
MOSFET Symbol

A MOSFET has 3 terminals, the Drain, Gate and Source.

The simple way to explain the operation of the MOSFET is though the relationship between the Drain-Source resistance and the Gate-Source voltage.

When there is no voltage difference between the Gate and Source, the Drain-Source resistance is large. As we increase voltage across the Gate and Source, the resistance across the Drain and Source decreases.

There is some physics which explains this phenomena, which you can read about here.

Relays

Relays are electro-mechanical components that are used to control electrical connection. They have two states: not activated (open, no electrical connection) and activated (closed, electrical connection exists).

Applying certain voltages to the input terminals, changes the state.

Note that we are using a solid state relay, which does not use mechanical components but functions very similar to a Mechanical relay.

Application to the Separation Mechanism

Circuit Diagram of the Seperation Mechanism, consisting of a 6V power source, a number of resistors, the SSR Relay, and 15V power source and the Nichrome wire.
Separation Mechanism Circuit Diagram

The Raspberry Pi operates on active low. That is, the default state of the GPIO Pin is high (outputting 3.3V), only when the DTMF sequence is received, the GPIO Pin is activated and turns low (0V).

  • When high, the Gate-Source Voltage is 3.3V causing the Drain-Source resistance to be small, which is treated as a short circuit. The output to the Solid State relay (below the 1k resistor) will be connected to 0V, which will not activate the relay.
  • When low, the Gate-Source Voltage will be 0V causing the Drain-Source resistance to be large, which is treated as an open circuit. The output to the Solid State Relay will be connected to the 6V battery through the 1k Resistor, which will activate the relay.

Activating the relay supplies 15V through the Nichrome wire (which is wrapped around the rope). This heats the Nichrome Wire, reaching temperatures high enough to melt through the rope.

Conclusion

Despite sounding simple, remotely burning a rope encompasses many areas of knowledge, Software, Electrical and Telecommunications. There are plans of future iteration to utilise Arduino and GPS coordinates to force separation. If you want to read more about the Balloon, you can read about it on the Balloon Page.

If you are interested in space, join BLUEsat by following the instructions here.

We actively ensure that every thing BLUEsat does and encourages remains legal. It is important to – as mentioned earlier – read, understand and follow the laws regarding radio transmission in your area and country.